At the present time, the major bonding systems being used in the manufacture of composite products of wood still utilize resin condensates of formaldehyde with urea, melamine or with phenol or other phenolics. As a result of general economic conditions and especially the recently increased cost of petrochemicals, there is a continuous pressure to reduce the cost involved in employing these resins in the manufacture of wood products. In addition, urea-formaldehyde resin releases formaldehyde which is carcinogenic and an eye and mucosa irritant, thus creating a possible health problem.
Adhesives based on carbohydrates, such as starches or sugars, which are converted to binders by various means, have also been suggested and have been adopted to some extent. Carbohydrate binders have an important advantage in lower production costs and zero formaldehyde emission from products. On the other hand, synthetic resin binders based on the condensation of formaldehyde with urea, melamine or phenol have a significant advantage over carbohydrate binders in low curing temperature and short curing times, which are important production cost affecting factors.
Urea and melamine-formaldehyde resins cure at 212.degree. F. in 13 to 45 seconds, depending on the formulation. Phenolformaldehyde resins cure at 220.degree. F. in about 90 seconds. However, carbohydrate adhesives normally need temperatures of at least 280.degree. F. to cure and the minimum cure time is about two minutes. The higher curing temperatures and longer curing times of carbohydrate binders can be reduced by using more efficient heat transfer systems such as steam injection pressing, as disclosed in Stofko U.S. Pat. No. 4,357,194, but application of this heating system requires sophisticated presses and more capital investment (also see Stofko U.S. Pat. Nos. 4,409,170 and 4,504,205). Thus, an improved binding system employing carbohydrates is disclosed in Stofko U.S. Pat. No. 4,357,194, which further teaches the addition of lignin or phenolics containing lignin to the carbohydrate bonding system.
There is also a large body of art directed to the manufacture of resins which are the condensation polymers of carbohydrates, formaldehyde and a member selected from the group consisting of urea and melamine. However, none of such art has been directed to and claims (a) wood bonding using a mixture of urea formaldehyde resin or melamine formaldehyde resin and sugars by transforming them to binders in situ -on wood surface-in a hot press; (b) a substantial replacement of urea formaldehyde or melamine formaldehyde resins by sugars resulting in a significant reduction of formaldehyde emission while maintaining or improving the quality of bonding; and (c) a significant reduction of cost of bonding which is inherent in the present invention.
For examples of such art see U.S. Pat. Nos. 2,252,725; 3,284,381 and 1,949,831. Thus, Ford in U.S. Pat. No. 1,949,831 proposes a process for producing durable resinous or plastic substances of a saccharide, urea and formaldehyde by reacting first 63.15% of a saccharide with 18.95% of formaldehyde and 2.1% of hexamethylenetetramine at temperatures below 100.degree. C., followed by reacting a product of the first reaction with 15.79% urea at temperatures of 100.degree.-150.degree. C. The final product is a white plastic powder suitable for molding in a press; there is no suggestion of the use of this product for bonding wood. In his related patent, 1,949,832, Ford proposes reacting the condensation product of saccharide and formaldehyde with phthalic anhydride instead of with urea, the ratio of anhydride and saccharide being about 1:1, to again produce a moldable plastic powder.
Hickey et al in U.S. Pat. No. 3,284,381 propose an improvement of starch adhesive for corrugated paperboard, by using amylose instead of starch. In order to increase the water-resistance, a quantity of 0.33-2.7% of urea and 0.65-2.7% paraformaldehyde are added to the amylose together with other ingredients. The preparation of this starch adhesive involves several steps of mixing and cooking.
U.S. Pat. No. 2,252,725 describes a process for producing a glucose-phenol-formaldehyde resin in which glucose is substituted for about 30% of the phenol. Other attempts also have been made to produce a sucrose-phenol-formaldehyde resin; for example, the International Sugar Institute has conducted research and has issued a research report on the preparation of sucrose-phenol-formaldehyde resin for the manufacture of plywood.
Other prior proposals of some interest include the Olix U.S. Pat. No. 2,736,678, in which there is proposed the production of sugar-urea-formaldehyde resin by reacting 63.4% sugar with 18.88% formaldehyde at 100.degree. C., followed by reacting a product of the first reaction with 15.8% urea at the same temperature. The resultant resin is proposed as an ingredient in silica-clay-starch ester adhesive, the patentee indicating that the performance of such an adhesive is enhanced by adding a small quantity, i.e. 1-8%, of such sugar-urea-formaldehyde resin to the basic composition intended for the bonding of corrugated paperboard.
Bowen in U.S. Pat. No. 2,150,148 has proposed a binder for plywood manufacture composed of urea, formaldehyde and zinc chloride. This patent further indicates that if 0.43% of sucrose is added to the mixture before cooking the resin, some advantages are achieved over the conventional urea-formaldehyde resins.
The Hoffmann U.S. Pat. No. 3,984,275 proposes a binder formulation for corrugated paperboard composed of starch, polyvinylacetate and vinyl acetate-ethylene copolymer, the ratio of the starch to the other components being 1:1. For the purpose of increasing the water-resistance of such binder, a small amount, i.e. less than 1%, of either urea-formaldehyde or melamine-formaldehyde is added to the adhesive composition.
Another improvement of starch adhesive for corrugated paperboard appears in Bauer U.S. Pat. No. 3,019,120, where it is suggested to incorporate cyanamide or an alkaline salt thereof which improves the water-resistance of the starch adhesive, if the cyanamide is used as a supplementary reactant with urea-formaldehyde at high alkalinity. The urea formaldehyde represents only 0.5-5% of the total weight of the starch.
Christ in U.S. Pat. No. 3,076,772 proposes a process for producing urea-phenol-formaldehyde resin extended by sulfite spent liquor. A special urea-phenol-formaldehyde resin of molar ratio 1 part urea, 5 parts phenol, 12 parts formaldehyde and 1.25 parts sodium hydroxide was formulated to make it compatible with sulfite spent liquor. There was about 50% of urea-phenol-formaldehyde resin, 27.46% of lignosulfonic acid, 8.57% of reducing sugars and small quantities of other substances in the final product.
An analysis of these prior disclosures show that they can be classified into two groups. In the first of these groups, resins or plastic substances have been produced of either sugar-urea-formaldehyde or sugar-phenol-formaldehyde. The basic feature of these processes is that new resins were prepared by cooking, using a definite sequence of reacting components by exposing them to elevated temperature, pressure and time. Final products are indicated to be useful as binders or plastic materials for molding. The products have a ratio of sugars to formaldehyde and urea on the order of 63:19:16. In the formation of sugar-phenol-formaldehyde, about 30% of phenol was substituted by sucrose giving the ratio of phenol:sucrose:formaldehyde of 35:15:50; taking into account additional materials in the mixes, the sugars constitute about 8.5% of the total resin solids.
In the second group of such prior documents, small amounts of carbohydrates were used as ingredients in producing urea-formaldehyde resins, or small amounts of urea-formaldehyde resins were used as ingredients in carbohydrate binders. Thus, 0.43% sucrose was used in cooking the urea formaldehyde-zinc chloride resins according to Bowen U.S. Pat. No. 2,150,148; less than 1% of either urea or melamine formaldehyde resins were used in cooking the starch-polyvinyl acetate binder according to Hoffmann U.S. Pat. No. 3,984,275; and 0.33-5% of urea formaldehyde was used in the preparation of starch adhesives according to Hickey U.S. Pat. No. 3,284,381 and Bauer U.S. Pat. No. 3,019,120.
U.S. Pat. No. 4,397,756 to Lehmann discloses a method of reducing formaldehyde vapor emissions from particleboards bonded with urea formaldehyde resin by adding a mixture of urea and starch to urea formaldehyde resin.
U.S. Pat. No. 3,022,258, in the name of Berry, discloses an aqueous wood finishing composition of urea-formaldehyde condensate, sugar and solvent. The solids composition may range from 1:1 to 1:3 of sugar to resin solids. The composition serves as a filler-sealer and there is no indication that the composition would act as a bonding agent in the manufacture of solid wood products from wood particles, nor that the sugar has a bonding function and can replace a quantity of urea-formaldehyde condensate; instead, the sugar is said to aid in the dispersion of the resin, to serve as a filler, and also to aid in the sanding of the hardened film.
U.S. Pat. No. 3,562,060, in the name of Stevens, concerns only a typical phenoplast or aminoplast condensation resin, the novel aspect being the inclusion of an accelerator which is an hydroperoxide. Up to 20 weight percent of filler may be included, such as wood flour, walnut shell flour and the like, but these fillers are inert, not adhesive.
U.S. Pat. No. 2,773,848, in the name of Lindenfelser, concerns an aminoplast laminating resin for use in the manufacture of high pressure, decorative laminates of the FORMICA and NEVAMAR type. The patentee indicates that the inclusion of sugar had a tendency to eliminate the problem of laminate bleeding, i.e. a tendency for the resin material of the core assembly to migrate into the print sheet causing discloration; but that sugars are unsatisfactory because of inconsistent results. According to Lindenfelser, the problem is overcome by the use of an alpha alkyl-D-glucoside as an additive. Again, there is no indication that such a resin will serve to adequately bond wood particles to make a satisfactory particleboard product, or that the glucoside can serve as a replacement for a portion of the aminoplast without diminution of the adhesive properties.